The design of a control system for foil-borne operation of a hydrofoil craft depends, in part, upon feedback information regarding the height of the craft above the water, or, alternatively, foil depth. This feedback is obtained from a height sensor which operates on the same basic principle as a pulsed radar system except that ultrasonic frequencies are used instead of microwave frequencies. Ultrasonic height sensors of this type have exhibited a sensitivity to noise impulses, the extent of the problem varying from one design to another depending on the sensor operating frequency and other internal design features, and the energy spectrum and duration of the noise impulses. If noise impulses of this type are not eliminated, the resulting change in craft height will typically vary from a foot or so (perceived as a "bump") to broaching of the forward foil, followed by slamming of the hull into the water. As can be appreciated, the effectiveness of the hydrofoil craft is degraded accordingly.
In the past, there has been no satisfactory means for solving the problem of noise impulses in height sensors of this type. Some existing designs utilize a "range gate" which rejects all return echoes except those received within some predetermined window positioned about the last processed return. This provides some noise rejection capability, but the design still processes as valid any noise occurring while the range gate is open. Furthermore, each time the range gate blanking is changed, the range gate is repositioned to a shorter range. As a result, interference continuing over several cycles of operation causes the craft to operate at a decreased foil submergence; and if it persists long enough, broaching can result.